In order to explore the variation of water requirement of crops with climate change in different seasons in the North China Plain, seasonal response of reference crop evapotranspiration (ET_o) to key climatic variables in the North China Plain was analyzed. First, the FAO56 Penman-Monteith equation was used to calculate ET_o at 48 weather stations over the study area from 1960 to 2012. Second, the interannual changing trend of ET_o and its key climatic influential factors, i.e. air temperature （T）, sunshine hours （n）, wind speed （u） and relative humidity (RH), were analyzed for different seasons. Sensitivity analysis had been done on ET_o sensitivity to the four climatic variables respectively. Finally, contributions of these climatic factors to the change in ET_o were analyzed based on the sensitivity of ET_o to these climatic factors and relative interannual changes in these climate factors. It was revealed that ET_o from 1960 to 2012 showed a downward changing trend in all the seasons. T increased while n, u, and RH decreased. ET_o was found to have a positive sensitivity to T, n, and u and a negative sensitivity to RH. The maximum sensitivity of ET_o to T and the maximum sensitivity of ET_o to n appeared in the summer, while the maximum sensitivity of ET_o to u and the maximum sensitivity of ET_o to RH appeared in the winter. It was found that the decline of ET_o in the spring, autumn and winter was mainly affected by the decrease in u, and the decrease of ET_o in the summer was mainly attributed to the decrease of n.